Electro-acoustic armature transducer

ABSTRACT

A balanced armature electro-acoustic transducer has small permanent magnets on its armature and on the pole-pieces with each armature magnet&#39;&#39;s poles facing like poles on the polepieces. Thus equal centering forces are set up. The arrangement uses rare-earth/cobalt alloys, which have a very high coercivity.

United States Patent 1 1 Cragg ELECTRO-ACOUSTIC ARMATURE TRANSDUCER [75] Inventor: William Donald Cragg, Harlow,

England [73] Assignee: International Standard Electric Corporation, New York, NY.

[22] Filed: Nov. 21, 1972 [21] Appl. No.: 308,381

[30] Foreign Application Priority Data Dec. 23, 1971 Great Britain 59930/71 [52] US. Cl 179/119 A, 179/117 A, 179/115 A, 335/231 [51] Int. Cl H04! 11/00 '[58] Field of Search ..179/114 R, 114 A, 115 R,

179/115 A, 117, 119 R, 119 A; 335/231; 148/103 Sept. 17, 1974 [56] References Cited UNITED STATES PATENTS 1,559,158 10/1925 Butcher et a1. 179/114 A 1,761,745 6/1930 Pomeroy 179/115 A McCracke I ..335/231 Benz 148/103 Primary Examiner1(ath1een H. Claffy Assistant Examiner-Thomas L. Kundert Attorney, Agent, or Firm-John T. OHalloran; Menotti J. Lombardi, Jr.

[5 7 ABSTRACT A balanced armature electro-acoustic transducer has small permanent magnets on its armature and on the pole-pieces with each armature magnets poles facing like poles on the pole-pieces. Thus equal centering forces are set up. The arrangement uses rareear'th/cobalt alloys, which have a very high coercivity.

3 Claims, 3 Drawing Figures 1 ELECTRO-ACOUSTIC ARMATURE TRANSDUCER The present invention relatesto an elfectro-acoustic transducer of the so-calle dmoving-iron:type,i.e. to one in which. the armature which drives (in the case of a loudspeaker orxa telephone earpiece) or is drivenby (in the case of a microphone) the diaphragm is a-member of aferro-magnetic material.

SUMMARY OF INVENTION According-to the present inventionthere isprovided anelectro-acoustic transducer inwhich an armature of amagnetically soft material is pivotally mounted so as to drive ,,on to be drivenby adiaphragrminwhich. the

armature has .atleast one of its endsalocated between two pole-pieces, in which each said pole-piece i'sv to microphones, loudspeakers and telephone earpieces,

andthat'a fully balanced-armature arrangement can'be usedinwhi'ch the arrnatureiscentrally. pivoted. Insuch acase the-armaturecarries-amagnet at each of'its ends, each such: magnet, cooperating with two stationary magnets, as set out. above.

BRIEF DESCRIPTION OF DRAWINGS" Embodiments of the invention will"now be described with reference to the accompanying drawings, in which:

FIG. 1' shows for explanatory purposes a conventionalrocking armature transducer;

FIG. 2 is an-explanatory-diag'ram of one embodiment of thepresent invention; and

FIG; 3 is'a slightlymore detailed representationzof a second embodiment:oftheinvention- DESCRIPTION. OF THE .P-REF ERRED EMBODIMENTS In one conventional rocking-armaturetransducer, a

permanent magnet 1' is provided -with'two pole-piece members 2, 3,v each of' which has twopole-pieces as shown. Between thesepole-pieces there is-a pivoted armature 4 toone end'of whichis :coupled adiaphragm 5. Coilsaremounted on the pole+piecemembers 2 and 3. In the case. of.atelephone earpiece, the application of speech currents tothe coils causesthe armature 4 to rock aboutitscentralpivot andthus to drive the diaphragrn 5. .The transducer has certain practicallir'nita- *tions-in:that'ifithearmature is deflected too far it is liable-to stick (or freeze) to the pole-pieces,,and that to keep itin acentralaposition a highrstiffnessisneeded in its pivotbearing.

The basic construction ofone form ofifloating armature transducer according to-the invention is shown in FIG. 2. In this arrangement, the armature 10 carries smallmagnetsll, l2 atzeach of its ends, and (as inthe deviceof FIG. l)is.coupled to a diaphragm13;.The

magnet 11- is between two similar: magnets .14 and 15,

while'the-magnet 12 is between two similar magnets 16 and 17;With the magnets poledasindicatedin FIGL. 2, it will be seen that the repulsive forces between the magnets causes the-armature to float freely in a central position.

The armature l0 is softly magnetic and has a coil wound on it to carry a signal current. If this current is in-sucha direction (as shown) as to make the left-hand end of the armature of a South polarity, then the lefthand endof the armature 10 moves upwards towards the magnet 14. When the current reverts to zero the armature, by virtue of the respulsive-forces on*it ,.is.immediately re-centralized. Thus it will be seen that with an alternating current in the coil, the armature oscillates 1 in sympathy with that current to drive the armature 13.

As long as'the alternating fluxes due to the signal are small compared with the steady flux, the movement is linearly related to the input electrical signal.

The magnets used are rare-earth cobalt magnets whose coercivity is of the order of 10,000 oersted; compared with which Alnico V has a coercivity of about 500. Thus the magnets can be small, and in the case of one transducer using magnets of a Samariumcobalt alloy they were 1mm cubes. With such an arrangement it is possible to obtain a floating armature with alow stiffness. The central magnet/armature arrangement is highly stablesince any deflection closes the spacing between magnetic. poles. of like polarity, which increases the centering force. Thus it is possible toobtain anarmature mounting of nearly zero stiffness,

so that. the overall acoustic impedanceof the transducer can be reduced until itis comparable-with theear impedance or with the air-'loadedi impedance. in the case of a microphone.

The magnets themselvesform the pole-pieces, and since they work in a demagnetizing'field'they can exist in isolation without the usualsoft magnetic circuit; As will be seen below, however, the latter can still be used where this is convenient. As will be seen from the above discussion the magnets can be'very small, i.e. 1 mm cubes, so that their dimensions are comparable with'the dimensions of the other parts of the device,

If the magnetic fields of the device shown schematically in FIG. 2 are plotted, it willbe found that at each end of the armature one. gap simulates aunipolar South pole while the other gap simulates a unipolar North pole.

When the arrangement is used as a microphone, it willbe seen that a diaphragmwill deflect the armature from its central position, with the result that flux is switched-through the-armature in one direction or the other, to give an induced current in the coil.

The transducer shown in FIG. 3 has a softiron return path 20 to the center of which is pivoted'anarmature 21, the left-hand end of which drives(or is driven by) a diaphragm. The armature and also the ends of the retum path 20 carry small magnets-poled as shown so that magnetically speaking the device of FIG. 3 is half that of FIG. 2, but with a soft ironreturn path.

The armature is laminated with the outer laminae bent as shown into horns as at 22, 23 aimed at the two air gaps. The coil is wound about-the armature 21. In this arrangement the'central magnet provides positivestiffness to the armature vibrations, .while the outer laminae act like a conventionalbalanced armature giving negative stiffness. The dimensions are such that over small excursions from a central position the addition of the positive and negative stiffnesse's approach zero, and that for larger excursions positive stiffness exceeds negative stiffness.

Note that in FlG.,3, the coils can be mounted on the v within said field, and a diaphragm responsive to the movement of said armature-wherein theimprovement.

comprises:

an armature of laminated construction having a central lamina which carries a permanent magnet, two 1 other lamina one on each side of said central lamina, wherein each'said other-lamina has onegof its ends formed t'o'i a horn-like portion extending "to wards said magnetic field.

a v. 1 2. A transducer as claimed in claim 1, and in which the coil embracesthe armature. l

3. A transducer according to claim 1 wherein said magnet is comprised of rare-earth cobalt alloys having a-coerc ivity in the order of 10,000 oersteds. 

1. An electro-acoustic transducer of the type having an armature pivotably mounted within a magnetic field, an electromagnetic coil for moving said armature within said field, and a diaphragm responsive to the movement of said armature wherein the improvement comprises: an armature of laminated construction having a central lamina which carries a permanent magnet, two other lamina one on each side of said central lamina, wherein each said other lamina has one of its ends formed to a horn-like portion extending towards said magnetic field.
 2. A transducer as claimed in claim 1, and in which the coil embraces the armature.
 3. A transducer according to claim 1 wherein said magnet is comprised of rare-earch cobalt alloys having a coercivity in the order of 10,000 oersteds. 